Beekeepers in a national survey reported that 23.2 percent of their colonies were lost over the winter of 2013-14. The latest numbers released May 15 were better than the 30.5 percent loss reported the previous winter, but higher than the 18.9 percent loss level that beekeepers say is acceptable for their economic sustainability.

USDA Secretary Tom Vilsack said the losses are "still too high and there is still much more work to be done to stabilize bee populations." USDA has been conducting the annual voluntary survey with the University of Maryland since 2006. About 7,200 beekeepers responded for the latest report, which covered the period from October 2013 to April 2014.

There is no way to tell why the bees did slightly better this year, said Jeff Pettis, co-author of the survey and research leader of the Bee Research Laboratory in Beltsville, Maryland. Bee health is complicated and populations can fluctuate yearly because of various factors, he said, including viruses, parasites, lack of nutrition due to fewer pollen sources, and sub-lethal effects of pesticides that can weaken colonies and make them more vulnerable to damage.

Bees, birds and other insects are essential partners for farmers and ranchers and help produce much of the nation's food supply, said Vilsack. More than three-fourths of the world's flowering plants rely on pollinators to reproduce, he said, and "healthy pollinator populations are critical to the continued economic well-being of agricultural producers."

A complete analysis of the bee survey data will be published later this year.